Roll compacting of metal powders



United States Patent fifice 3,318,986 ROLL COMPACTlNG F METAL POWDERSRobert William Fraser and Vilnis Silins, Fort Saskatche- Wan, Alberta,and Bud William Kushnir, Edmonton, Aiberta, Canada, assignors to GordonSherritt Mines Limited, Toronto, Ontario, Canada, a company of Canada NoDrawing. Filed May 13, 1964, Ser. No. 367,220 4 Claims. (Cl. 264111)This invention relates to methods by which metal and metal coatedparticles are roll compacted into metallic strips and sheets and, moreparticularly, to improvements in such methods which permit the rollcompaction of fine metallic powders of normally poor flowcharacteristics.

Methods are known in the powder metallurgy art by means of which metaland metal coated composite powders can be compacted to form sheets andstrips. Generally, the powder is first compacted into a green strip orsheet by feeding it, usually at a predetermined, uniform rate,vertically into the roll gap of a roll compacting unit comprised of apair of oppositely positioned, horizontally disposed pressure rolls. Theroll diameter and roll gap are selected to provide a self-supporting,partially densified green strip of the desired thickness."

The green strip is generally sintered and hot rolled and otherwise hotand/ or cold worked to produce a finished, or semi-finished metallicproduct of substantially 100% density.

Problems are encountered when these methods are applied to metal powdershav'mg poor flow characteristics as such powders either will not flowinto the roll gap at all or they do not flow uniformly and green stripof very poor quality is produced.

The flow characteristics of metal powders are generally measured byreference to the standard flow test for metal powders which is set outin ASTM Standards 1961, part 3, page 1047 (ASTM designation B213-48).Some metal powders do not flow at all under the conditions of this test;such powders are said to have a flow rate of infinity. Other powders mayhave measurable flow rates according to the standard test butnonetheless do not have flow characteristics which enable them to beuniformly fed by conventional means to the roll gap of a roll compactingunit.

Although the particle size is not the only characteristic which affectsthe flow characteristics of metal powders, it is generally found thatmetal and metal coated powders having a particle size up to aboutmicrons, Fisher sub-sieve size, have very high or infinite flow ratesaccording to the standard test and do not fiow readily into the roll gapof conventional powder rolling units which rely on a gravity feed from avertically arranged hopper. For the purpose of the present descriptionsuch powders will be referred to as fine powders and all particle sizesgiven in microns will be Fisher sub-sieve sizes.

Metal powders for roll compacting into strips are not always availablein the particle size range, size distribution and shape generally foundto result in flow characteristics most suitable for roll compacting. Insome cases, the process by which the powder is obtained may inherentlyproduce relatively fine metal particles having poor flowcharacteristics. Also, it is sometimes desirable to specifidfildfidfiPatented May 9, 1967 cally employ very fine particles for rollcompacting in order to produce wrought strip possessing certaindesirable qualities. For example, in the production of dispersionstrengthened metal strip from composite metal-metal oxide powders suchas nickel coated thoria, it is generally desirable to utilize finepowder in order to obtain uniform, close spaced distribution of thedispersed phase and, also, to ensure a fine, preferably less than 10microns, grain size in the matrix metal.

Previously efforts to overcome roll compacting mill feeding problemsresulting from poor flow characteristics in fine metal powders haveincluded expedients such as slowing down the roll speed; roughening thesurfaces of the rolls; force feeding powder to the roll gap; surroundingthe vicinity of the roll gap with an atmosphere of a gas of a lowerviscosity than air, such as hydrogen; maintaining a partial vacuum inthe vicinity of the roll gap; agglomerating the powder such as bysintering or other means; vibrating means on the hopper or like powderfeed mechanisms; and various combinations of the foregoing.

None of these methods has been found to provide an entirely adequatesolution to the problem of compacting fine metallic powders having poorflow characteristics.

'We have found that generally they are ineffective, un-

duly expensive, or impractical for commercial utilization.

We have found that the problems encountered in gravity feding finemetallic powders with poor fiow characteristics can be simply andeffectively overcome by mixing the powder with a predetermined quantityof water or a like inorganic or organic liquid which is capable of beingcompletely volatilized during the sintering operation to which compactedgreen strip is customarily subjected in subsequent processing steps. Theamount of liquid added must be sufiicient to change the flowcharacteristics of the powder from that dependent on the free flow ofthe powder particles to that dependent on mutual adhesion of theparticles.

The invention will be described in detail hereinafter with referencemainly to water as the liquid additive; however, it is to be understoodthat other liquids, such as naphtha, benzene, kerosene, glycol, and thelike volatilizable liquids may be equally suitable under propercircumstances, such as, for example, where oxidation of the powder bywater is a problem.

The precise amount of liquid to be added for any given powder dependsprimarily on the particle size of the powder, although particle shapeand apparent density of the powder also have some affect. In general,the amount of liquid addition provided in accordance with this inventionis in the range of from about 1 to about 6 percent by weight. For mostfine powders, an amount within this range will cause mutual adhesiveforces to be set up between the particles comprising the powder. Theamount of liquid addition required for optimum results increases withdecreasing particle size. We have found, for example, that fine powdershaving a particle size in the range of about 1 micron to about 5 micronsgenerally require a water addition of about 2.5 to about 4.0 percent byweight, and powders in the 5 to 10 micron size range will require about1.0 to about 2.5 weight percent water addition.

The liquid additions provided in accordance with this invention do notfunction as a lubricant to increase the free flowing characteristics ofthe powder or to provide lubrication at the compression zone in the rollgap;

hesive forces between particles becomes too weak, and the powder ceasesto be drawn into the roll gap and the quality of the green strip rapidlydeteriorates. This problem is usually avoided by providing means, suchas rather, the effect is to change the flowability of the pow- 5 anabsorbent wiper pad or rubber or plastic scrapers, to der fromdependency on the free flowing characteristics continuously removeexcess moisture from the rolls. In of the powder particles under gravityto dependency on addition, where the rolling operation is continuous,the mutual adhesion of the particles. According to the presamount ofliquid initially added may be decreased someent invention, the criterionfor satisfactory roll compacwhat from that required to roll compactshort sections tion is thus no longer the ability of the powder to flowof the same powder to ensure that when equilibrium is evenly anduniformly into the roll gap under the inilureached, the liquid contentat the roll gap is at the optience of gravity but is the ability of thepowder partimum level for uniform feeding. cles in the roll gap to pullother particles down into the The invention is further illustrated bythe following roll gap through adhesive forces set up between theparexamples: ticles. This phenomenon is marked by very definite limExample 1 lts for any glven Powden An excess addltl'on In this example,a nickel-thoria composite powder, weakens the adhesive forces and toolittle will not set prepared for producing dispersion strengthenedWrought the necessary dheslve forces and both cases the nickel, wasutilized as the starting material. The powder result is that particlesare not drawn into the roll gap contained 985% nickel and 1.7 Weightpsrcent thoria; at all, or, at best, insufiicient amounts enter the rollgap the Fisher Subsieve Size was microns and the appar t0 ProduceStrong, dense green P- Thus, for $355 ent density was 1.66 grams percubic centimetre. The factory T011 compaction, it is essfintial that theliquid powder initially had a flow rate of infinity under the conditionbe carefully controlled within the upper and lower ditio of th t d dASTM fl t t, i i did not limits applicable for any given powder. It issimple for flow at all, Sa l of thi owde r ix d ith one ordinarilyskilled in the art to determine, by a few various amounts of water andcompacted in a laboratory trial runs conducted in accordance with theteachings compacting mill. The mill was gravity fed from a feed set outherein, the precise optimum liquid requirements hopper positioned abovethe rolls and having a feed throat fo a given owder, extendingvertically into the roll gap. The diameter of The liquid is mixed withthe powder by any conventhe rolls was 6 inches and the roll gap in everycase was tional mixing procedure, such as by means of double set at0.005 inch. The results of these tests are shown cone blenders, whichensures uniform distribution of the in Table 1.

TABLE 1 Green Strip Properties Water Content, Thick- Weight/ wt. percentness, Density, unit/area, Strip quality in. percent gnL/in. (centre) 0.014 Only edges compacted. 0.5 0.19 Tail and edges compacted. 1 018 Sameas 0.5% but improved. 1. 5 .022 73 1.94 Complete strip but very brittle.2 022 74. 5 2. 23 Good continuous flexible strip. 2.5 77 2. 79 Do. 3.025 78.5 2. 73 Do. 3. 5 77. 5 2. 71 Do. 4 .026 76. 5 2.77 Small holesin strip near the tail 5 026 75 2 72 L r iger and more numerous holes instrip near tail end of sample. 6 024 75 2 52 Low density area and holesnear edge of strip. 7 Broken pieces of strip.

liquid through the powder. The moistened powder is These resultsillustrate the effect of water additions on then fed into the gravityfeed hopper of the compacting the compacting behaviour of thenickel-thoria composite mill and roll compacted in the conventionalmanner. powder under gravity feed conditions. There are definite Thechemical properties and the precise physical proplimits (2.0 to 3.5weight percent in this specific example) erties of the liquid additiveare not critical factors. In which define the optimum water content forproduction general, all that is essential is that the liquid flowsreadof sound green strip. Above and below this region, poor ily at thenormal mixing and feeding temperatures and quality strip is produced.The results also indicate the that it be volatilizable and non-residueforming at the effect of water addition on strip thickness, density andtemperatures at which the green strip is sintered. For weight per unitarea, all of which increase with increasing economic reasons water isthe preferred liquid additive, water content to maximum values at theoptimum water but where oxidation of the metal powder is a problemcontent then decrease with further increases. in water an equivalentamount of another liquid such as naphtha content. will be equallyeffective. It can :be noted further that with this particular pow- Afactor which must be taken into account in carrying der with wateradditions of 4% and higher the defects out the method of this inventionis that during compactin the strip tend to occur near the tail end whichindicates ing, some liquid is squeezed out of the moistened powder thatthere was a build-up of excess moisture in the roll and tends to buildup in the roll gap and on the surfaces gap after a period of time, thuscausing the quality of the of the rolls. If this build-up becomesexcessive, the adstrip to deteriorate.

Example 2 In this example, the starting material consisted of samples ofpure nickel powder of various sizes. The compacting and mixingprocedures were the same as those of Example 1. The results are set outin Table 2.

TABLE 2 Green Strip Properties Water Powder Content,

Size, wt. Thiek- Density, Wt./unit microns percent ness, percent area,Strip quality in. theogrn./in. retical (centre) 1 1 Powder would notflow and did not compact. 2. 7 1 015 51. 1 17 Continuous strip, veryweak. 2 .015 58.0 1 27 Good strip. 0 011 1 Continuous strip, very week.4.0 1 .015 64. 5 1. 42 Weak strip.

3 .016 68.5 1. 60 Good. 5 015 64.0 1. Holes in tail. 0 .011 59. 5 0. 96Weak but continuous strip. 5 2 2 .015 71.8 1.57 Good.

6 010 70.2 1.64 Good (hole in tail). 8 1 Poor, pieces of strip only. 0.013 64. 3 1. 22 Continuous strip, weak. 2 .016 74.0 1.73 (100d. 7 3 4018 74. 0 1. 94 Good, weak towards tail. 6 .016 70.7 1. 65 Good, holesin extreme tai 8 Holes across strip.

Example 3 TABLE 3 Water Addi- Strip Thicktion, wt. ness, inc StripQuality percent 1. 0 0.32 Edges compacted only. 2.0 9.32 Very weakcentre. 3.0 0.38 Good green strip. 4. 0 0. 43 D0. 5.0 0. 43 Do.

All of the above green strips were badly oxidized. 5.0 weight percentnaphtha was substituted for water in another sample of the same powder.Good quality green strip 0.40 inch thick and free from any oxidation wasproduced.

Although the present invention has been described in conjunction withthe preferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

What we claim as new and desire to protect by Letters Patent of theUnited States is:

1. A method for forming finely divided metal and metal coated compositeparticles of a size within the range of about 1 micron to about 5microns into self-sustaining green strip by feeding said particles intothe roll gap defined between a pair of oppositely positioned rolls ofdrawn into the roll gap of said roll compacting unit, and rolling theliquid containing particles to produce green strip.

2. The method according to claim 1 in which the liquid is one of thegroup consisting of water, naphtha, benzene, kerosene, and glycol.

3. A method for forming finely divided metal and metal coated compositeparticles of a size within the range of about 1 micron to about 5microns into self-sustaining green strip by feeding said particles intothe roll gap defined between a pair of oppositely positioned rolls ofthe roll compacting unit which comprises mixing said particles prior tocompaction with water in amount within the range of about 2.5 to 4.0% byweight, said amount being sufiicient to cause said particles to adheretogether such that they are drawn into the roll gap of said rollcompacting unit, and rolling the water containing particles to producegreen strip.

4. The method of forming finely divided nickel and nickel coatedcomposite particles of a size within the range of about 1 micron toabout 5 microns into selfsustaining green strip by gravity feeding saidparticles into the roll gap defined between a pair of oppositelypositioned rolls of a roll compacting unit which comprises mixing saidparticles prior to feeding with water in amount within the range ofabout 1.0 to 4.0% by weight, said amount being sufiicient to cause saidparticles to adhere together such that they are drawn into the roll gapof said compacting unit, and rolling the water containing particles toproduce green strip.

References Cited by the Examiner UNITED STATES PATENTS 2,363,575 11/1944Lamatter et al. 23-208 2,869,215 1/1959 Smith 264-56 ROBERT F. WHITE,Primary Examiner. F. S. WHISENHUNT, I. R. HALL, Assistant Examiners.

1. A METHOD FOR FORMING FINELY DIVIDED METAL AND METAL COATED COMPOSITEPARTICLES OF A SIZE WITHIN THE RANGE OF ABOUT 1 MICRON TO ABOUT 5MICRONS INTO SELF-SUSTAINING GREEN STRIP BY FEEDING SAID JPARTICLESINTHE ROLL GAP DEFINED BETWEEN A PAIR OF OPPOSITELY POSITIONED ROLLS OFA ROLL COMPACTING UNIT WHICH COMPRISES MIXING SAID PARTICLES PRIOR TOCOMPACTIN WITH A VOLATIZABLE, NON-RESIDUE FORMING LIQUID IN AMOUNTWITHIN THE RANGE OF ABOUT 1 TO ABOUT 6 WEIGHT PERCENT, SAID AMOUNT BEINGSUFFICIENT TO CAUSE SAID PARTICLES TO ADHERE TOGETHER SUCH THAT THEY AREDRAWN INTO THE ROLL GAP OF SAID ROLL COMPACTING UNIT, AND ROLLING THELIQUID CONTAINING PARTICLES TO PRODUCE GREEN STRIP.